The present invention relates to a steering apparatus equipped with a transmission ratio adjusting mechanism (hereinafter, referred to as a transmission ratio adjustable steering apparatus) that is preferably employed in automotive vehicles and is capable of changing a transmission ratio of a steered angle of a steerable wheel or tire to a steering angle of a steering wheel controlled or manipulated by a driver or an operator.
Some of advanced automotive steering control apparatuses employ a transmission ratio changing mechanism, such as Variable Gear Ratio Steering (VGRS), that can change a steering angle conversion ratio (i.e. a transmission ratio of a steered angle of a steerable wheel to a steering angle of a steering wheel). In general, it is desirable to change the steering angle conversion ratio with reference to vehicle driving conditions including a vehicle traveling speed. For example, when a vehicle is traveling at a higher speed, the steering angle conversion ratio should be decreased to avoid rapid change in the steered angle of the wheel relative to a steering angle of the steering wheel controlled or manipulated by a driver because the traveling performance of the vehicle is stabilized at higher speeds.
On the other hand, when the vehicle is traveling at a lower speed, it is desirable to increase the steering angle conversion ratio in order to reduce a required minimum steering angle of the steering wheel when a driver must manage to put his/her vehicle into a garage or into a limited parking space and accordingly in order to reduce the burden of a driver who must control or manipulate the steering wheel.
Conventionally, many motor type steering mechanisms are widely used for adjusting the steering angle conversion ratio because they are excellent in independently rotating and driving a steered wheel shaft. More specifically, an angle detecting section is provided to detect a steering angle of a steering wheel controlled or manipulated by a driver. The steering angle conversion ratio is determined based on the detected steering angle of the steering wheel and actual vehicle driving conditions. Through computer processing, a finally required wheel steering angle (i.e. target wheel angle) is calculated based on the determined steering angle conversion ratio. Then, the steered wheel shaft, mechanically separated from the steering shaft of the steering wheel, is rotated or driven by the motor so as to adjust the angle of the steerable wheel to the target wheel angle.
According to this kind of steering control system, the steered wheel shaft is follow-up controlled so as to accurately agree with a rotation of the steering shaft. More specifically, the rotational speed of a steered wheel shaft driving motor is adjusted in accordance with a difference between an actual rotational angle of the steered wheel shaft (i.e. actual wheel steering angle) and a target wheel steering angle. The follow-up control should be performed speedily so that the actual steering shaft angle agrees with a target steering angle as quick as possible. However, to avoid undesirable overshoot phenomenon occurring during a final stage of this kind of follow-up control, it is necessary to sufficiently decelerate and accurately control a rotational speed of the motor. On the other hand, a driver may suddenly turn the steering wheel to avoid immediate danger. In such a case, the motor must rotate at a very high speed.
Furthermore, a transmission ratio changing mechanism may include a locking apparatus, according to which the transmission ratio is mechanically fixed to a predetermined value in case of failure occurring in the transmission ratio changing mechanism. For example, a locking apparatus includes an arch-shaped locking arm swingably supported to a motor housing of the transmission ratio changing mechanism. And, a disc-shaped lock holder is fixed around a rotor shaft. The locking arm has a projection selectively engageable with a recess of this lock holder. When the projection of the locking arm engages with the rotor shaft, the relative rotation between the motor housing and the rotor shaft is locked. Accordingly, in a case that the engine is stopped or in an event that an excessive force is applied to the transmission ratio changing mechanism, the locking apparatus is brought into a locked condition to surely maintain the accurate and regulated relationship between the steering angle of the steering wheel and a steered angle of the steerable wheel. (Refer to Japanese Patent Application Laid-open No. 11-34894).
However, according to the transmission ratio adjustable steering apparatus disclosed in the above-described prior art, there is a drawback such that the locking arm and the lock holder make uncomfortable noises when they engage to bring the locking apparatus into the lock condition.